1. Field of the Invention
This invention generally relates to methods for reducing the susceptibility of a peak search to signal noise. Certain embodiments relate to determining a peak value of a pulse based on the largest sum of sample values rather than the largest sample value.
2. Description of the Related Art
Generally, flow cytometers provide measurements of fluorescence intensity of laser excited polystyrene beads as they pass linearly through a flow chamber. In some systems, there are four measurements that are performed: the level of light scattered by a bead at 90 degrees to the excitation source, two measurements of fluorescence used to determine the bead “identity,” and a third fluorescence measurement typically used to quantify a chemical reaction of interest. Each of the three fluorescent measurements is made at a different wavelength.
In applications such as flow cytometry, the detectors generate a number of output signals or “samples” that make up a single pulse. Each pulse is generally attributable to one event such as, in the case of flow cytometry, illumination of one bead. As the event progresses, the light measured from scattering and/or fluorescence emission by the bead increases as the bead enters the beam of light, reaches a maximum at the center of the beam, and tapers to a nominal value as it leaves the beam. In this manner, a number of output signals will be generated over time, and the samples which make up each individual pulse may be determined. Generally, identifying the samples which belong to a single pulse involves determining a peak value of the pulse. A number of samples occurring before and after the peak value is then summed.
However, there are several disadvantages to the current methods for determining the peak value of the pulse. For example, a peak value is generally determined by identifying the largest sample in a set of samples. The pulse is assumed to be symmetrical about the peak value. A predetermined number of samples occurring before and after the peak value are then summed. However, when a signal being digitized includes noise, it is possible that a noise spike in the vicinity of the desired pulse may be misidentified as the pulse peak value thereby causing the summation to include incorrect sample data (i.e., samples that are actually baseline values).
Accordingly, it may be advantageous to develop a method for identifying the peak value of a pulse, which may or may not include noise, such that the summation may accurately reflect the pulse.